Anchors for stranded pretensioned members



Dec. 31, 1963 K. H. MIDDENDORF 23,115,727

ANCHQRS FOR STRANDED PRETENSIONED MEMBERS Filed Nov. 29, 1957 INVENTOR/ Karl H. flwnaorf' bd/P. A1941,

A TTORNE Y- United States Patent Ofiice 3,115,727 ANCHR FQR STRANDED PRETENSEONED MEMBERS Karl E. Midd-endorl', Costa Mesa, Calif., assignor to The Preston Corporation, a corporation of Texas Filed Nov. 29, 1957, Ser. No. 699,634 6 Claims. (til. 50-128) My invention relates to anchors for tension members for prestressing concrete, and particularly to an anchor for a stranded pretension member.

It has become common practice to use stranded cable for pretensioning concrete in this country, and in order to obtain more rapid re-use of forms in casting beds, it has become the practice to release the stranded tension members from the means fior applying the tension thereto in approximately twenty-four hours or less after the concrete has been poured. This has been done under the assumption that the convolutions or twists of the wire providing a spirally ribbed outer contour of the stranded tension member offered so much more resistance than a smooth wire to shifting of the wire with respect to the concrete surrounding the same that complete bonding was eifected between the twisted strand and the wire and stresses exerted by the wire on the concrete would be completely transferred to the concrete due to this bond between the wire and the concrete.

Recently doubts have arisen on the part of those skilled in the art that such a bond between the concrete and the wire was established and maintained, particularly along the stranded tension member adjacent the ends of a structural member, such as a beam, in which such tension members are mounted. It is doubtful that full bond is effected until a point about six feet from the end of the stranded tension member is reached. As shearing stresses, which are greatest generally at the end or at the supporting point of a structural member, are presumed to be resisted by compression induced by the prestressing of the concrete, this questionable bonding of the stranded tension member and the concrete is particularly serious with respect to counteracting shearing stresses.

It is well known that cast iron, cast steel, or malleable cast iron, or similar products, have shearing values in relation to fully cured concrete in a ratio of approximately 500 to 1, and with concrete cured -for only twentyfour hours this ratio would probably be 1000 to 1. As the bond between the concrete and the stranded tension member is relatively weak because of the relatively weak and friable concrete convolutions formed around the twisted wire of the stranded tension member when it is cast around the same, it is a purpose of my invention to provide means for anchoring the stranded tension member to the concrete in such a manner that the stresses exerted on the concrete by the tensioned member are transferred thereto, through an anchoring member, instead of directly through the spiral ribs on the pretensioned member to the concrete surrounding the same to put the concrete under compression.

It is a further purpose of my invention to accomplish the above referred to anchoring of the stranded tension member to the concrete by means of a member of any suitable material that will have a relatively high shearing value with respect to concrete, such as the materials above referred to, which inter-fits with the spirally ribbed surface of the stranded tension member so that it is interlocked therewith by means of the interfitting spiral grooves and ribs of the tension member and the anchor member. Said anchoring member is preferably of a generally annular character and of considerably greater outside diameter than the stranded tension member on which it is mounted, and is of such a character that after the concrete has been cast around the same and cured substantially twenty-four 2 hours, it will be held against rotation in the body of concrete. Preferably the outside diameter of the anchor member is approximately three times the outside diameter of the stranded tension member.

It is a further purpose of my invention to provide an anchoring member for stranded tension members, that has a relatively large area, on a side face thereof, which is placed in direct compression against the concrete, thus transferring the compression to a large area of the concrete body portion of the structural member extending around the stranded tension member for a considerable distance along its length, the zone in which this compression is created being of a conical character extending from the anchoring member lengthwise of the structural member, with the axis of the conical compression zone of the structural member substantially on the axis of the stranded tensioned member.

Preferably my anchoring member for a stranded tension member is split, comprising a plurality of portions formed for ready application to the stranded tension member. First the strand is tensioned and then the anchor member is applied around the stranded tensioned member and is secured in position by means of a suitable clip or similar means for holding the portions thereof in position on the stranded tensioned member, and after this has been done the concrete is poured. The attempt of the tensioned member to return to its unelongated condition then places shear on the spirally ribbed portions of my anchoring member that are seated between the spirally ribbed portions of the stranded tension member, which would tend to cause the anchoring member to rotate about the axis of the stranded tension member. It is a further purpose of my invention to provide suitable means for preventing rotation of the anchoring member about the axis of the stranded tensioned member in the body of concrete, as by making the annular anchoring member of a shape other than truly circular. As a result, the anchoring member and tensioned member can not move relatively, as the tensioned member would only be capable of moving lengthwise of itself relative to the anchor, if it were free to rotate relative thereto, and as a result the tension exerted by the tensioned member, which causes the stranded member to tend to move lengthwise in the body of concrete, is transferred, through the interengaging spiral formations on the walls of the opening for the tension member in the anchor and the complementary spiral formations on the tension member, to the concrete, which is put under compression.

Other objects and advantages of my invention will appear as the description of the drawings proceeds. I desire to have it understood, however, that I do not intend to limit myself to the particular details shown or described, except as defined in the claims.

In the drawings:

FIG. 1 is a fragmentary top plan view of a structural member, showing one form of my anchoring member applied to a stranded tension member.

FIG. 2 is a fragmentary vertical longitudinal sectional view through said structural member, showing said anchoring member applied to the stranded tension member, said anchoring and tension members being shown in elevation.

FIG. 3 is a transverse sectional view through the anchoring member and tension member, showing the same as it would appear prior to the pouring of the concrete.

FIG. 4 is an inside view in elevation, taken on the line 44 of FIG. 3, of the anchoring member detached from the tension member, and

FIG. 5 is a fragmentary transverse sectional view through the anchoring member and tension member on an enlarged scale.

Referring in detail to the drawings, my improved anchor member is shown as being applied to a stranded tension member 5 mounted in a body of concrete 6-, which may be in the form of a beam or other structural member having an end 7. Said stranded tension member 5 extends generally lenghtwise of the structural member and, being made up of Wires that are twisted around each other, has obliquely extending adjacent spiral ribs 8 thereon defining spiral grooves between the same, as will be obvious from FIG. 5, the grooves resulting from the circular cross sectional character of the twisted wires that form the stranded tension member 5.

My improved anchor member is shown as being of a disk-like character and as made up of a pair of members 9 that are substantially duplicates, and which have side faces that have flat central portions 10 and conical portions 11 that extend from the central portions thereof to the peripheral portion of each thereof. A tension member receiving recess is provided in each of said body portions 9, said recess being of a generally concave character and the recesses of the two body portions or members 9 forming a tension member receiving opening extending through the disk-like anchor member from one flat face 10 thereof to the other flat face lb thereof. Said concave recesses in said members 9 are provided with concave spiral grooves 12, which are shaped to closely receive the convex spiral ribs 8 of the stranded tension member 5, and which define tapering spirally extending ribs 13 located between the same, which seat in the tapering spirally extending grooves that are defined by the strands of the tension member 5. Portions of one of said grooves for receiving one of the strands of the tension member on one side of the core thereof and of the strand opposite the same are formed partly in one of the members 9 and partly in the other member 9, as indicated at 12 in FIG. 5.

Said body members 9 are provided with flat inclined faces 14 and 15, which are located opposite each other when the two halves 9 of the anchor member are assembled in the position shown in FIGS. 1, 2i and 3. The depth of the recesses 8 is such that the faces 14 and 15 of the halves 9 of the anchor member will be slightly spaced from each other when a U-shaped holding clip 16 is assembled with said members 9' to hold the same in engagement with the stranded tension member, as shown in FIG. 3. Said U-shaped clip preferably is made of spring wire material and has outturned end portions 17, the legs 18 thereof being received in grooves 19 in the members 9.

The grooves 19 are provided in the outer periphery of the two-part disk-like anchor member made up of the halves or portions 9. As will be obvious from FIG. 3, the anchor member, when assembled and held in assembled position by means of the clip is of somewhat greater thickness across the diameter thereof that intersects the grooves 19 than across the diameter thereof that is at right angles thereto. As shown in FIG. 3, the curved peripheral portions 2d gradually increase in radius of curvature toward the faces 14 and 15 of the members 9 and become substantially flat faced portions 21 adjacent the flat faces 14 and 115 of the portions 9 of said anchor member. As a result, when the body of concrete 6, in which the anchor member is embedded, has set sufficiently, the anchor member will be held against any rotation about the axis of the stranded tensioned member 5. It will also be obvious that if the anchor member is held against such rotation by the concrete, it will be fixed in position relative to the stranded anchor member 5.

In the use of my improved anchor member, the stranded tensioned member 5 is first pretensioned, and while it is in its pretensioned condition the two halves 9 of the anchor member are placed around the same land the clips in put in position to hold the anchor member in assembled relation with respect to the stranded tensioned member. The spiral ribs 8 on the tensioned member will then be seated snugly in the grooves 12 and 12 of the anchor member and the spiral ribs -13 on the anchor member will be seated firmly in the spiral grooves between the strands of the tensioned member 5. After the concrete has been poured around the stranded tensioned members that have the anchor members above described placed in position thereon, as above pointed out, the same is allowed to set sufficiently that the anchor member will not revolve or rotate in the concrete, twenty-four hours setting being ordinarily sufllcient for this purpose, whereupon =the means that has applied the tension to the tensioned member 5 is released. The tendency of the tensioned member 5 to shorten thereupon, causes a stress to be exerted on the tensioned member 5, acting from left to right in FIGS. 1 and 2. This is resisted by the anchor, the resistance being due to the fact that the anchor member can not rotate and is fixed in position in the concrete. As a result, a shearing stress is applied to the ribbed portions 13 lying between the grooves 12 of the anchor member, but because of the much greater resistance to shear of the material of which the anchor member is made, this is many times the resistance to such shear that the con crete has, that is seated between the spiral ribs on the stranded tensioned member 5, and is more than sufficient to prevent any damage to the interlock between. the ten sioned member and the anchor member.

The forces that are acting on the anchor member are transferred through the body portions 9 of the anchor member to the conical face 11 thereof and the flat face 10 thereof on the right hand side thereof, as viewed in FIGS. 1 and 2. As a result, the concrete body portion 6 is put under compression over a conical Zone bounded substantially by the lines 22, thus putting the portion of the structural member that is subjected to shear, because of its being mounted on a supporting surface, near its end 7 under the desired compression to resist the shearing strain applied thereto with a desired factor of safety,

What I claim is: v

1. In a prestressed concrete structural member having a concrete body portion, a stranded wire tension member having a spirally ribbed surface embedded in said body portion to extend longitudinally of said structural mem ber with an end portion thereof projecting from an end of said structural member, and auxiliary compression means mounted on said tension member to project from the periphery thereof within said concrete body portion adjacent said end of said structural member, said compression means having a split body portion of a material having a high resistance to shear, said body portion 66in prising a plurality of member and yielding means engaging said members to hold the same in position in surrounding relation to said tension member, said split body portion having a tension member receiving opening therethrough, the wall of said opening having spirally extend ing ribs thereon projecting into said opening and engag ing between the spiral rib of said tension member,- whereby stressing said tension member will tend to rotate said auxiliary compression means about the axis of said opening, said split body portion having peripheral SUI" faces eccentric to said axis to hold the same from rotation; about said axis in said concrete body portion, said auxil-' iary compression means having a conical side wall facing away from said end of said structural member and upon stressing of said tension member defining a conical compression zone in said concrete body portion extending from said compression means in surrounding relation to said tension member, said Zone widening from said auxiliary compression means in a direction away from said end of said structural member.

2. In a prestressed concrete structural member having a concrete body portion, a stranded wire tension member having a spirally ribbed surface embedded in said body portion to extend longitudinally of said structural member with an end portion thereof projecting from an end of said structural member, and auxiliary compression means mounted on said tension member to project from the periphery thereof within said concrete body portion adjacent said end of said structural member, said compression means having a split body portion of a material having la high resistance to shear, said body portion comprising a plurality of members and yielding means engaging said members to hold the same in position in surrounding relation to said tension member, said split body portion having a tension member receiving opening therethrough, the wall of said opening having a plurality of spirally extending concave grooves therein receiving the spiral ribs of said tension member, whereby stressing said tension member will tend to rotate said auxiliary compression means about the axis of said opening, said split body portion having peripheral surfaces eccentric to said axis to hold the same from rotation about said axis in said concrete body portion, said auxiliary compression means having a conical side wall facing away from said end of said structural member and upon stressing of said tension member defining a conical compression zone in said concrete body portion extending from said compression means in surrounding relation to said tension member, said zone widening from said auxiliary compression means in a direction away from said end of said structural member.

3. In a prestressed concrete structural member having a concrete body portion, a stranded Wire tension member having a spirally ribbed surface embedded in said body portion to extend longitudinally of said structural member with an end portion thereof projecting from an end of said structural member, and auxiliary compression means mounted on said tension member to project from the periphery thereof within said concrete body portion adjacent said end of said structural mermber, said compression means having a split annular body portion of a metal having a high resistance to shear, said body portion comprising a plurality of members and yielding means engaging said members to hold the same in position in surrounding relation to said tension member, said split body portion having a tension member receiving opening therethrough, the wall of said opening having spiral formations thereon extending longitudinally of said opening and interfitting with the spiral ribs on said tension member, whereby stressing said tension member will tend to rotate said auxiliary compression means about the axis of said opening, said split body portion having peripheral surfaces eccentric to said axis to hold the same from rota tion about said axis in said concrete body portion, said auxiliary compression means having a conical side wall facing away from said end of said structural member and upon stressing of said tension member defining a conical compression zone in said concrete body portion extending from said compression means in surrounding relation to said tension member, said zone Widening from said auxiliary compression means in a direction away from said end of said structural member.

4. In a prestressed concrete structural member having a concrete body portion, a stranded wire tension member having a spirally ribbed surface embedded in said body portion to extend longitudinally of said structural member with an end portion thereof projecting from an end of said structural member, and auxiliary compression means mounted on said tension member to project from the periphery thereof within said concrete body portion adjacent said end of said structural member, said compression means having a split annular body portion of a metal having a high resistance to shear, said body portion comprising a plurality of members and yielding means engaging said members to hold the same in position in surrounding relation to said tension member, said split body portion having a tension member receiving opening therethrough, the wall of said opening having spiral formations thereon extending longitudinally of said opening and interfitting with the spiral ribs on said tension member, whereby stressing said tension member Will tend to rotate said auxiliary compression means about the axis of said opening, said split body portion having peripheral surfaces eccentric to said axis to hold the same from rotation about said axis in said concrete body portion, said auxiliary compression means having a conical side wall facing away from said end of said structural member and upon stressing of said tension member defining a conical compression zone in said concrete body portion extending from said compression means in surrounding relation to said tension member, said zone widening from said auxiliary compression means in a direction away from said end of said structural member, said conical side wall extending continuously from adjacent one end of said opening to the periphery of said annular body portion.

5. In a prestressed concrete structural member having a concrete body portion, a stranded wire tension member having a spirally ribbed surface embedded in said body portion to extend longitudinally of said structural member with an end portion thereof projecting from an end of said structural member, and auxiliary compression means mounted on said tension member to project from the periphery thereof Within said concrete body portion adjacent said end of said structural member, said compression means having a split body portion of a material having a high resistance to shear, said body portion comprising a plurality of members and yielding means engaging said members to hold the same in position in surrounding relation to said tension member, said split body portion having a tension member receiving opening therethrough, the wall of said opening having spirally extending ribs thereon projecting into said opening and engaging between the spiral ribs of said tension member, whereby stressing said tension member will tend to rotate said auxiliary compression means about the axis of said opening, said split body portion having peripheral surfaces eccentric to said axis to hold the same from rotation about said axis in said concrete body portion and curved peripheral surfaces extending parallel to the axis of said opening, said auxiliary compression means having a conical side Wall facing away from said end of said structural member and upon stressing of said tension member defining a conical compression zone in said concrete body portion extending from said compression means in surrounding relation to said tension member, said zone widening from said auxiliary compression means in a direction away from said end of said structural member.

6. In a prestressed concrete structural member having a concrete body portion, a stranded wire tension member having a spirally ribbed surface embedded in said body portion to extend longitudinally of said structural member with an end portion thereof projecting from an end of said structural member, and auxiliary compression means mounted on said tension member to project from the periphery thereof Within said concrete body portion adjacent said end of said structural member, said compression means having a split annular body portion of a metal having a high resistance to shear, said body portion comprising a plurality of members and yielding means engaging said members to hold the same in position in surrounding relation to said tension member, said split body portion being approximately three times the diameter of said tension member and having a tension member receiving opening therethrough, the wall of said opening having spiral formations thereon extending longitudinally of said opening and interfitting with the spiral ribs on said tension member, whereby stressing said tension member will tend to rotate said auxiliary compression means about the axis of said opening, said split body portion having peripheral surfaces eccentric to said axis to hold the same from rotation about said axis in said concrete body portion and curved peripheral surfaces extending parallel to the axis of said opening, said auxiliary compression means having a conical side wall facing away from said end of said structural member and upon stressing of said tension member defining a conical compression zone in said concrete body portion extending from said compression means in 7 surrounding relation to said tension member, said zone widening from said auxiliary compression means in a direction away from said end of said structural member, said conical side Wall extending continuously from adjacent one end or" said opening to the periphery of said annular body portion.

References (Iited in the file of this patent UNITED STATES PATENTS Cryer Nov. 21, 1939 Feltenberger Mar. 26, 1940 

1. IN A PRESTRESSED CONCRETE STRUCTURAL MEMBER HAVING A CONCRETE BODY PORTION, A STRANDED WIRE TENSION MEMBER HAVING A SPIRALLY RIBBED SURFACE EMBEDDED IN SAID BODY PORTION TO EXTEND LONGITUDINALLY OF SAID STRUCTURAL MEMBER WITH AN END PORTION THEREOF PROJECTING FROM AN END OF SAID STRUCTURAL MEMBER, AND AUXILIARY COMPRESSION MEANS MOUNTED ON SAID TENSION MEMBER TO PROJECT FROM THE PERIPHERY THEREOF WITHIN SAID CONCRETE BODY PORTION ADJACENT SAID END OF SAID STRUCTURAL MEMBER, SAID COMPRESSION MEANS HAVING A SPLIT BODY PORTION OF A MATERIAL HAVING A HIGH RESISTANCE TO SHEAR, SAID BODY PORTION COMPRISING A PLURALITY OF MEMBERS AND YIELDING MEANS ENGAGING SAID MEMBERS TO HOLD THE SAME IN POSITION IN SURROUNDING RELATION TO SAID TENSION MEMBER, SAID SPLIT BODY PORTION HAVING A TENSION MEMBER RECEIVING OPENING THERETHROUGH, THE WALL OF SAID OPENING HAVING SPIRALLY EXTENDING RIBS THEREON PROJECTING INTO SAID OPENING AND ENGAGING BETWEEN THE SPIRAL RIBS OF SAID TENSION MEMBER, WHEREBY STRESSING SAID TENSION MEMBER WILL TEND TO ROTATE SAID AUXILIARY COMPRESSION MEANS ABOUT THE AXIS OF SAID OPENING, SAID SPLIT BODY PORTION HAVING PERIPHERAL SURFACES ECCENTRIC TO SAID AXIS TO HOLD THE SAME FROM ROTATION ABOUT SAID AXIS IN SAID CONCRETE BODY PORTION, SAID AUXILIARY COMPRESSION MEANS HAVING A CONICAL SIDE WALL FACING AWAY FROM SAID END OF SAID STRUCTURAL MEMBER AND UPON STRESSING OF SAID TENSION MEMBER DEFINING A CONICAL COMPRESSION ZONE IN SAID CONCRETE BODY PORTION EXTENDING FROM SAID COMPRESSION MEANS IN SURROUNDING RELATION TO SAID TENSION MEMBER, SAID ZONE WIDENING FROM SAID AUXILIARY COMPRESSION MEANS IN A DIRECTION AWAY FROM SAID END OF SAID STRUCTURAL MEMBER. 